1. Field of the Invention
This invention relates to an improved resin mold or pattern used for making, for example, a vehicular panel or sheet.
2. Description of the Related Art
A known resin mold for making a vehicular panel or sheet by vacuum or press forming is disclosed in, for example, Japanese Patent Publication No. HEI-07-106576 entitled xe2x80x9cRESIN MOLDxe2x80x9d. The mold has an electrically conductive intermediate layer of a polymer formed on a thermosetting resin, and a metal plating layer formed on the surface of the intermediate layer by, for example, nickel plating. The metal plating layer is required to have a sufficiently large thickness ranging, for example, from 50 to 300 microns to ensure durability. The formation of such a thick layer, however, requires a lot of time which is undesirable from the standpoint of productivity in a job of mold manufacture. Moreover, the layer is not satisfactory in wear resistance, since it is formed simply by plating the surface of the intermediate layer with, for example, nickel, chromium, copper or zinc. It is likely to get worn relatively soon if the mold is used for vacuum or press forming vehicular panels or sheets repeatedly.
Therefore, there is also known a resin mold which does not have any such plating layer, but is satisfactory in wear resistance, and can be made in a relatively short time. The mold has a wall structure as fragmentarily shown in FIG. 14. The mold 100 comprises a main body 105 and a surface layer 101 formed on its surface. The main body 105 is formed from a thermosetting epoxy resin 107 containing aluminum particles 106. The surface layer 101 is formed from a thermosetting epoxy resin 103 containing aluminum flakes 102. The surface layer 101 has a molding surface 108 having recesses 108a and protrusions 108b by which an uneven, or grained surface is formed on the surface 110a of a sheet 110, as described below.
When the mold 100 is used for press forming the sheet 110, it is cooled to 50xc2x0 C., while the sheet 110 is heated to 180xc2x0 C. Then, its surface layer 101 is pressed against the sheet 110 to form it into a desired shape. The heat of the sheet 110 is transferred to the surface layer 101, and main body 105 of the mold 100 to raise its temperature to 120xc2x0 C. Then, the mold 100 is opened, the sheet is removed therefrom, and after it is cooled to 50xc2x0 C. again, the process as described above is repeated for another sheet. For raising the productivity of the press forming operation, therefore, it is important to cool the mold 100 from 120xc2x0 C. to 50xc2x0 C. quickly and shorten the time for which it has to wait to be cooled. The aluminum particles 106 in the main body 105 of the mold 100 are, however, not uniform in size, but include large and small particles. Therefore, the aluminum particles 106 are not joined together in a regular mold, and there are, for example, small particles 106a not contacting medium-sized or large particles 106b or 106c. The failure of those small particles 106a to contact medium-sized or large particles 106b or 106c makes it difficult for the main body 105 to have a high thermal conductivity. Thus, the mold 100 has to wait for so long a time to be cooled from 120xc2x0 C. to 50xc2x0 C. and become ready again that it is difficult to obtain a high productivity in any press forming operation. Moreover, the main body 105 is low in strength, since small aluminum particles 106a are not securely held by medium-sized or large ones 106b or 106c. 
The surface layer 101 contains a large amount of thermosetting resin 103, since it fills the interstices between the aluminum flakes 102 densely. On the other hand, the main body 105 contains a small amount of thermosetting resin 107, as compared with that of the resin 103 in the surface layer 101, since it has a porous structure formed by the aluminum particles 106 not uniform in size, but creating open pores 109 which are required for the evacuation of the cavity of the mold. As the thermosetting resin 103 or 107 has a higher coefficient of linear expansion (hereinafter referred to as coefficient of thermal expansion) than aluminum, the surface layer 101, or main body 105 has a coefficient of thermal expansion depending on the amount of the thermosetting resin 103 or 107 which it contains. As the surface layer 101 contains a larger amount of resin, it has a higher coefficient of thermal expansion, but its thermal expansion is restricted by the main body 105. As a result, a high stress is produced in the surface layer 101 and this eventually causes it to crack.
It is, therefore, a first object of this invention to provide a resin mold which can be made in a short time and is excellent in wear resistance.
It is a second object of this invention to provide a resin mold which is improved in thermal conductivity and strength.
It is a third object of this invention to provide a resin mold having a sufficiently small difference in coefficient of thermal expansion between its layers to prevent any cracking of its surface layer.
According to a first aspect of this invention, there is provided a resin mold which comprises a main body formed from a thermosetting resin containing metal particles and having a molding surface, and a surface layer formed on the molding surface from a thermosetting resin and containing fine particles of silicon carbide.
This surface layer can be formed simply by thermosetting the resin. As it does not call for any plating, the mold can be made in a short time. The silicon carbide in the surface layer is high in hardness and wear resistance, and makes it resistant to wear.
The resin forming the surface layer preferably contains 50 to 80% by weight of silicon carbide. If the proportion of silicon carbide is less than 50% by weight, the surface layer may not be satisfactorily resistant to wear, as its entire surface may not be completely covered with silicon carbide. If its proportion exceeds 80% by weight, the surface layer is difficult to form, since the proportion of the resin is too small for its satisfactory fluidity.
The main body has a porous structure defined by open spaces formed between the adjoining metal particles and enabling the mold to be evacuated.
According to a second aspect of this invention, there is provided a resin mold which comprises a main body formed by metal particles held together with a thermosetting resin and having a molding surface, and a surface layer formed on its molding surface from a thermosetting resin, the metal particles being spherical and substantially equal in diameter, every adjoining three of those particles contacting each other.
The mold has a high thermal conductivity owing to the spherical metal particles having substantially the same diameter and therefore maintaining contact between every two adjoining ones. When the mold is used for shaping, for example, a sheet, the heat transferred from the sheet to the mold can be dissipated effectively, so that the mold can be cooled in a short time. Moreover, every two adjoining metal particles are held fast by each other and thereby contribute to improving the strength of the mold.
The spherical metal particles preferably have a diameter of 0.3 to 3.0 mm. If their diameter is smaller than 0.3 mm, the open spaces formed between every three adjoining particles are too small for evacuation purposes. If their diameter exceeds 3.0 mm, the open spaces are too large for the mold to maintain a high thermal conductivity.
According to a third aspect of this invention, there is provided a resin mold which comprises a main body formed from a thermosetting resin containing metal particles and having a molding surface, an intermediate layer formed on its molding surface from a thermosetting resin containing pieces of a nonwoven metal fabric, and a surface layer formed on the intermediate layer from a thermosetting resin, the intermediate layer having a coefficient of thermal expansion which is lower than that of the surface layer and higher than that of the main body.
The intermediate layer having a coefficient of thermal expansion as defined above is intended for mitigating a large difference in the degree of thermal expansion which may arise between the surface layer and the main body. The thermal expansion of the surface layer is not restricted by the intermediate layer supporting it, nor is the thermal expansion of the intermediate layer restricted by the main body supporting it. Therefore, it is possible to lessen any stress produced in not only the surface layer, but also the intermediate layer, and thereby prevent their cracking. The nonwoven metal fabric which the intermediate layer contains in the resin gives the mold a high thermal conductivity.
The main body preferably has open spaces formed by every two adjoining metal particles, through which the mold can be evacuated.